The present invention relates to the field of semiconductor lithography, in particular to a method and apparatus for coating a substrate with a polymer solution and for drying the applied coating. More specifically, the invention relates to the formation of photo-, electron-, and X-ray resists on the surfaces of semiconductor wafers.
Resists are organic or inorganic materials which are applied either directly onto the surface of semiconductor substrate or onto the surface of a topological layer preformed on the substrate for the formation of a selected latent image of a pattern which later is turned into a functional layer of a chip to be produced. In a majority of cases, the materials applied on the semiconductor substrate are organic materials. There exist a great variety of such materials. Among them, tens of polymer compositions are commercially used as polymer-type resists. Types and properties of some of them are described, by Wayne M. Moreau in: xe2x80x9cSemiconductor Lithography. Principles, Practices, and Materialsxe2x80x9d, Plenum Press, New York, London, 1988.
In the process of development of the latent image formed in the resist as a result of exposure to light or other radiant energy, the exposed or non-exposed areas are removed by subsequent etching processes to form the so called mask having a configuration of the aforementioned pattern. The aforementioned process is known as semiconductor lithography. The mask produced by the semiconductor lithography plays a crucially important role for protecting the masked areas during processing of opened areas in subsequent processes such as doping by ion implantation, coating in lift-off lithography, etching, etc. Films from which masks are formed are normally have a thickness within the range of from fractions of micron to several microns.
It is understood that protective property of the mask is one of the most important factors in the quality of the entire chip manufacturing process.
The properties of the mask, in turn, to a great extent depend on technological operations used in the manufacture of the masking film, such as application of a coating material on a substrate from a solution, uniform spreading of the applied material over the substrate, and drying of the applied coating for the formation of a coating film.
Quality of the masking films is determined by such factors as uniformity of thickness and properties, presence of defects such as pinholes, surface cracks, structural nonuniformity caused by foreign particles, etc., and adhesion.
It is understood that processes used for the manufacture of the mask should exclude formation of the aforementioned defects and combination with high adhesion of the film to the substrate.
It is obvious that a main process that determines the final quality of the mask is drying. In the context of the present invention, the term xe2x80x9cdryingxe2x80x9d means removal of the solvent from the liquid polymer coating applied onto the surface of the substrate. It is understood that physical and chemical processes, which accompany removal of the solvent from resist also, may lead to conversions in the polymer itself. Such conversion, however, occur at temperatures higher than the temperature of drying. It should be noted in this connection, that the step of drying can be divided into a process of drying itself at a temperature that does not cause the aforementioned conversions and a process of baking at a temperature that is maintained to cause such conversion as hardening.
More specifically, drying out of a material includes the processes of penetration of the solvent from the polymer itself into free volumes in the polymer, such as voids, microcracks, or air bubbles with subsequent transition of the solvent from the liquid state into a vapor phase a gaseous phase (vapor). Kinetic characteristics (time behavior) of the above-indicated processes determine the mechanism of removal of the solvent from the coating.
The initial stage of drying, which is normally carried out at room temperatures, is characterized by a high content of the solvent in the polymer coating. As the coating becomes dry, the yield of the solvent into a gaseous phase is retarded, whereby the rate of drying is reduced. It is known, however, that by holding the polymer coating only at room temperatures, it is impossible to obtain high values of protective properties such as adhesion, defect-free condition, etc.
Therefore, for obtaining the above properties, drying should include a high-temperature stage (i.e., the stage at a temperature above room temperature but below the glass transition temperature for the polymer xe2x88x92Tg. After completion of the phase separation at the first stage of drying with the formation of a polymer matrix that contains the solvent, the increase in the process temperature accelerates diffusion of the solvent into the polymer (as a rule, a constant of diffusion depends on the temperature exponentially). As a result, the solvent is rapidly removed from the polymer matrix. Increase in the polymer temperature also decreases its viscosity, which in addition to the aforementioned accelerated diffusion, leads to a decrease in internal stress. This, in turn, affects uniformity of adhesion over the coating area. It is understood, however, that the high-temperature stage of drying should not exceed the level at which such undesirable thermodestruction or thermopolymerization may occur.
It is known that the initial stage of drying of a polymer at an increased temperature is accompanied by a sharp increase in the rate of solvent removal. After having reached its maximum, this rate then drops to zero.
As has been mentioned above, evaporation of the solvent from the external surface of the coating occurs in parallel with a phase transition (evaporation) into free volumes of the polymer matrix, such as microcracks, voids, etc. This process is known as internal vapor formation.
It has been proven experimentally that connection exists between internal vapor formation and protective properties, for example, of a photoresist coating. Many factors influence kinetic characteristics of the process of the internal formation. The following are examples of these factors: concentration of the solvent in the polymer, solvent vapor pressure, geometry of microcavities, density of distribution of microcracks and microcavities in the coating volume, coefficient of diffusion of the solvent, viscosity and surface tension of the polymer coating, and temperature of the coating.
The increase in the rate of internal vapor formation leads to an increase in concentration of defects and to a decrease in adhesion of the coating to the substrate. These phenomena are caused by an increase in the gas pressure of solvent vapor in microcracks and microcavities and by subsequent opening of the aforementioned microcracks and microcavities to the interface between the substrate and the coating film and to the external surface of the film.
It is obvious that surface microcracks as well as microcavities and microcracks located near the surface of the coating film affect adhesive and protective properties of the coating film to a lesser degree than those located inside the film and on the interface between the coating and the substrate. It is understood that aggregation of such defects distributed across the film cross section may lead to the formation of pinholes in the coating film.
At the first stage of drying the internal vapor generation in the protective coating with a high concentration of the solvent does not essentially affects protective properties of the polymer coating. This is explained by favorable conditions for removal of the solvent at this stage of the drying, such as a relatively high rate of diffusion of the solvent molecules in the polymer film and high mobility of the polymer molecules enhancing closing of microcracks and microcavities.
Decrease in the concentration of he solvent is accompanied by an increase in the viscosity of the polymer in the coating film and decrease in planarization ability (as used herein the planarization ability is an ability of the polymer to create a flat surface and to heal irregularities of edges of the opened microcracks).
It should be noted that microcracks and microcavities in the coating film greatly vary in their shape and dimensions and that the smaller these defects, the better properties of the final coating.
The factor preventing the action of internal vapor formation and suppressing the propagation of gas microcracks is an excessive external pressure in combination with heating of the coating film.
An attempt has been made to improve properties of the coating film by a method comprising the steps of: retaining the polymer coating at room temperature for a time interval from 20 sec to 1 hour; heating of the coating at an increased pressure sufficient for suppressing propagation of microcracks into the coating and for deteriorating its properties; and cooling the treated coating. See an article by V. P. Lavrischev, V. A. Peremychtchev in: xe2x80x9cStudy of mechanism of removing the solvent from the photoresist filmxe2x80x9d, 1975 Electronics, issue 5 (53), pages 58-65).
However, the entire process was conducted in one and the same chamber, and this did not allow to eliminate the phenomenon of internal vapor formation. Therefore the method described above did not allow to produce a defect free product. Furthermore, this method does not ensure adequate adhesion of the coating film to the substrate, which shortens the service life of the polymer coating. Both drawbacks are initiated by the process of propagation of microcracks of the coating during its drying. This has been confirmed by experiments conducted with the use of an apparatus described in xe2x80x9cElectronic industryxe2x80x9d No. 5 (77), pages 50-52, 1979, Moscow, xe2x80x9cUnit for forming photoresist coatings AFF-2xe2x80x9d, by V. V. Anufrienko, V. I. Osnin, V. A. Peremychtchev, V. L. Sanderov, V. N. Tsarev.
The aforementioned apparatus has a sealed working chamber with a heater connected to a loading chamber via an air-tight damper on one side and to an unloading chamber via an air-tight damper on the other side. The working chamber can be connected via an appropriate shut-off valve system to a high-pressure main.
In such a device the excessive pressure is built up at the stage of holding the polymer coating at elevated temperatures. In this case, building-up of the excessive pressure in a high-temperature chamber is possible only after loading the substrate into the chamber and closing the loading hatch with an air-tight damper. A disadvantage of the aforementioned device is that the high pressure is released while the substrate is still hot and development of microdefects is still possible.
Furthermore, even an insignificant time shift between the high-pressure process and the high-temperature process may result in an instant reaction of the film to deviations in the drying conditions with the formation of the cracks. This is because the film is very thin and can be instantly overheated under normal pressure if the application of high pressure and high temperature are not synchronized.
U.S. Pat. No. 5,361,515 issued to Peremychtchev on Nov. 8, 1994 discloses a method and apparatus for drying the protective polymer coating applied onto the surface of a substrate article from solution. The process described in the aforementioned U.S. patent is characterized by the fact that at the drying stage of holding the coating at room temperature, the action of excessive pressure precedes the raise of temperature, while at the stage of cooling the temperature drop precedes the release of high pressure.
The apparatus of U.S. Pat. No. 5,361,515 differs from the apparatus described above by a provision of two additional drying chambers located in front and behind the working chamber respectively. The front drying chamber, which is installed between the loading chamber and the working chamber, is intended for drying the coating at room temperature, while rear drying chamber is intended for cooling after drying at high temperature and high pressure in the main working chamber.
A main disadvantage of the invention of U.S. Pat. No. 5,361,515 is that the front and rear chambers have limited functional capabilities. More specifically, the front drying chamber, which determined initial stage of drying, makes it possible to conduct initial drying only in a strictly specified temperature range of 18xc2x0 C. to 28xc2x0 C. However, films formed prior to transfer to the main high-temperature and high-pressure chamber under the indicated temperature range, may have meso- and macroscopic nonuniformities. This is because isolation of the phase with low content of the solvent may occur already in the initial drying stage in the front chamber. This means that the in the coating film transferred to the main chamber the solvent may already have a nonuniform distribution. In other words, the coating film will contain inclusion of a solid phase, i.e., inclusions of the phase, which is harder than the rest of the coating material. Such clusterization takes place during polymerization even in a liquid phase.
In subsequent drying under high temperature and high pressure the aforementioned solid phase inclusions will serve as sources of concentration of stress, impair adhesion, and form microcracks and microcavities around the nuclei of the stress.
It is known that the film formation process has a very complicated mechanism, which depends not only on the temperature of drying but also on variation of temperature in time. Therefore it is very important to control the initial drying process in time. However, the apparatus of U.S. Pat. No. 5,361,515 does not allow such control.
The stage of cooling in the rear chamber after release of pressure is carried out via a contact-type cooler which does not allow quick and combined modes of cooling which may be required for obtaining a high quality coatings free of internal stress and microdefects in combination with high adhesive properties of the coating film.
It is an object of the invention to provide a method and apparatus, which allow initial drying in a wide temperature range with controlled temperature variation mode in the drying stage. Another object is to provide method and apparatus which allow quick and combined modes of cooling of the coating film at the cooling stage after release of high temperature and high pressure. Still another object is to provide coating films, which are free of defects caused by internal stress and microcavities.
An apparatus of the invention is intended for the formation of coating films on substrates and consists of a main high-temperature and high pressure drying chamber with two additional chambers located in front and behind the working chamber respectively. The front drying chamber, which is installed between the loading station and the working chamber, is intended for drying the coating at room temperature, while the rear drying chamber is intended for cooling after drying at high temperature and high pressure in the main working chamber. The front and rear chambers are provided with means for adjusting the respective drying and cooling processes by means of respective heating and cooling systems. This allows initial drying in a wider temperature range and final cooling under most optimum conditions.